1. Field of the Invention
The present invention relates generally to a liquid-crystal display. In particular, the invention relates to a liquid-crystal display with a wide viewing angle, and more particularly relates to a closed-cavity liquid-crystal display.
2. Description of Prior Art
Liquid-crystal displays (LCD) are widely used as the displays for portable computers and televisions since such displays are light, thin and energy-saving. At present, the most popular commercial LCDs are twist-nematic (hereinafter briefly referred to as TN) mode LCDs and super twist-nematic (STN) mode LCDs. The total twist angle of the molecules is normally 90 degrees for TN LCDs, and equal to or more than 180 degrees for STN LCDs. These types of LCDs are suitable for use as black/white gray scale displays. However, utilizing such LCDs as multiple gray-scale or full-color displays causes a severe problem of viewing angle dependency. That is because the optical paths for the incident light are different for different entrance angles, i.e., the reflective index of the liquid-crystal varies with the variation of entrance angle. Referring to FIG. 1a, which illustrates a cross section of a TN LCD, while a bias voltage is applied, the liquid-crystal molecules 5 incline at a tilt angle .phi.. The corresponding reflective index is n.sub.// or n.sub..perp. while the entrance angle is .theta. or -.theta.. The different reflective indices cause different optical paths; as a result, the optical transmittances are different. The contrast ratio is different for the same tilt angles (.+-..theta.). Furthermore, while watching the display panel along a tilt direction, the place that should be dark leaks light and hence the contrast ratio is degraded. Furthermore, the black and white of the intermediate gray scales are reversed. Thus, the picture can not be really reproduced. This problem becomes more severe while watching the display along its vertical direction. The total viewing angle of this case without gray scale reverse is merely about 40 degrees.
To solve the above problems, a display mode of divided domain or multi-domain of liquid-crystal molecules in one pixel is provided, which has been proven to have a viewing angle more than twice that of TN-LCD's. Referring to FIG. 1b, which illustrates the cross section of a multi-domain LCD which has two domains in one pixel, while a bias voltage exists, the tilt angles of the liquid-crystal molecules in the two domains are .phi. and -.phi., respectively. When the light is incident at a with a tilt angle .theta., the effective refractive index of the liquid-crystal is n.sub.//. When the light is incident at b with the tilt angle .theta., the effective refractive index of the liquid-crystal is n.sub..perp.. When the light is incident at a' with a tilt angle -.theta., the effective refractive index of the liquid-crystal is n.sub..perp.. When the light is incident at b' with the tilt angle -.theta., the effective refractive index of the liquid-crystal is n.sub.//. Therefore, the averaged intensity of the light incident with the angle .theta. is the same as that of the light incident with the angle -.theta.. That is, the contrast ratios are the same at both the tilt angle .theta. and the tilt angle -.theta.. Thus, the viewing angle is broadened. However, while fabricating such multi-domain liquid-crystal displays, a photolithography process has to be used to divide the multiple domains. Each domain needs to be treated by rubbing along a different direction. However, this rubbing process introduces impurities in the LCD cells and deteriorates the property of the LCD. The static electricity produced by rubbing the substrate tends to damage such devices as thin-film transistors, and thereby lower the yield.
Another known prior art is the axially symmetric aligned microcell (hereinafter referred to as ASM), which is disclosed in "Axially Symmetric Aligned Microcell (ASM) Mode: Electro-Optical Characteristics of New Display Mode with Excellent Wide Viewing Angle" by N. Yamada, Sokohzaki, F. Funada, K. Awane, in SID 95' Digest, p.575. The viewing angle without gray scale reverse is about .+-.60.degree. for an ASM liquid-crystal display. Refer to FIG. 1c, which illustrates the structure of an ASM liquid-crystal display. The fabrication of the ASM liquid-crystal display is accomplished by making the polymer 4 separate the liquid-crystal 3 into each pixel according to the phase separation of the polymer and the liquid-crystal. As shown in FIG. 1d, a set of polarizers whose transmitting axes are the X-axis and the Y-axis, respectively, are used to measure the contrast ratio at each viewing angle. As a result, the viewing angle .theta. is only 40.degree. when the azimuthal angle .phi. is 45.degree., 135.degree., 225.degree. or 315.degree.. Furthermore, it is difficult to fabricate an ASM liquid-crystal display since the upper electrode and the lower electrode thereof must be precisely aligned to make the liquid-crystal able to separate to each pixel.